An IEEE 802.16m system can support both a frequency division duplex (FDD) scheme that includes half-frequency division duplex (H-FDD) user equipment operation and a time division duplex (TDD) scheme. The IEEE 802.16m system uses an orthogonal frequency division multiplexing access (OFDMA) as a multiplex access mode in a downlink and an uplink. OFDMA parameters will be illustrated in Table 1 below.
TABLE 1Nominal Channel Bandwidth (MHz)578.751020Over-sampling Factor28/258/78/728/2528/25Sampling Frequency (MHz)5.681011.222.4FFT Size5121024102410242048Sub-Carrier Spacing (kHz)10.9375007.8125009.76562510.93750010.937500Useful Symbol Time Tu (μs)91.429128102.491.42991.429Cyclic PrefixSymbol Time Ts (μs)102.857144115.2102.857102.857(CP)FDDNumber of OFDM4834434848Tg = ⅛ Tusymbols per FrameIdle time (μs)62.85710446.4062.85762.857TDDNumber of OFDM4733424747symbols per FrameTTG + RTG (μs)165.714248161.6165.714165.714Cyclic PrefixSymbol Time Ts (μs)97.143136108.897.14397.143(CP)FDDNumber of OFDM5136455151Tg = 1/16 Tusymbols per FrameIdle time (μs)45.7110410445.7145.71TDDNumber of OFDM5035445050symbols per FrameTTG + RTG (μs)142.853240212.8142.853142.853Cyclic PrefixSymbol Time Ts (μs)114.286[TBD][TBD]114.286114.286(CP)FDDNumber of OFDM43[TBD][TBD]4343Tg = ¼ Tusymbols per FrameIdle time (μs)85.694[TBD][TBD]85.69485.694TDDNumber of OFDM42[TBD][TBD]4242symbols per FrameTTG + RTG (μs)199.98[TBD][TBD]199.98199.98
Hereinafter, a frame of the IEEE 802.16m system will be described in brief.
FIG. 1 is a diagram illustrating a basic frame in an IEEE 802.16m system.
Referring to FIG. 1, each superframe of 20 ms includes four same sized radio frames of 5 ms, and starts with a super frame header (SFH). If the same OFDMA parameters are used as illustrated in Table 1 at any one of channel bandwidths 5 MHz, 10 MHz, and 20 MHz, each radio frame of 5 ms includes eight subframes. One subframe can be allocated for downlink or uplink transmission. The first type subframe includes six OFDMA symbols, and the second type subframe includes seven OFDMA symbols, and the third type subframe includes five OFDMA symbols.
The basic frame can be applied to both the FDD scheme and the TDD scheme including H-FDD user equipment operation. In the TDD system, two switching points exist at each radio frame. The switching points can be defined in accordance with directional variation from the downlink to the uplink or from the uplink to the downlink.
The H-FDD user equipment can be included in the FDD system, and a frame is similar to a TDD frame in view of the H-FDD user equipment. However, downlink and uplink transmission occurs at two separate frequency bandwidths. Transmission gaps between the downlink and the uplink are required to switch transmitting and receiving circuits.
FIG. 2 is a diagram illustrating an example of a TDD frame having a downlink to uplink ratio of 5:3.
Referring to FIG. 2, it is supposed that an OFDMA symbol duration is 102.857 μs and has a cyclic prefix (CP) length corresponding to ⅛ of a useful symbol length Tu. In this case, the first type subframe and the third type subframe respectively have lengths of 0.617 ms and 0.514 ms. The last downlink subframe SF4 is a third type subframe. The transmit transition gap (TTG) and receive transition gap (RTG) are 105.714 μs and 60 μs, respectively. In other numerology, the number of subframes per frame may be different from the number of symbols within a subframe.
FIG. 3 is a diagram illustrating an example of a frame in an FDD mode.
Referring to FIG. 3, a base station that supports an FDD scheme can support a half-duplex user equipment and a full-duplex user equipment at the same time, wherein the half-duplex user equipment and the full-duplex user equipment are operated using the same radio frequency carrier. A user equipment that supports an FDD scheme should use any one of H-FDD scheme and FDD scheme. For both downlink transmission and uplink transmission, all subframes can be used. Downlink and uplink transmission can be divided in a frequency domain. One super frame is divided into four frames, wherein one frame includes eight subframes.
FIG. 4 is a diagram illustrating TDD and FDD frames having a CP length corresponding to 1/16 of a useful symbol length Tu.
Referring to FIG. 4, for channel bandwidths of 5 MHz, 10 MHz, and 20 MHz, a frame of the IEEE 802.16m system has five first type subframes and three second type subframes in an FDD scheme, and six first type subframes and two second type subframes in a TDD scheme, wherein the frame has a CP length corresponding to 1/16 of a useful symbol length Tu.
Supposing that an OFDMA symbol duration is 97.143 μs and has a CP length corresponding to 1/16 of a useful symbol length Tu, the first type subframe and the second type subframe have lengths of 0.583 ms and 0.680 ms, respectively. The TTG and the RTG are 82.853 μs and 60 μs, respectively. In other numerology, the number of subframes per frame may be different from the number of symbols within a subframe. In case of the FDD, a frame structure (the number of subframes, type, etc.) of an uplink should be the same as that of a downlink for each specific frame.
In the IEEE 802.16m system, when a frame having a CP length corresponding to ¼ of a useful symbol length is defined for bandwidths of 5, 10, 20, 7, and 8.75 MHz, a frame can be configured by a subframe smaller than a frame having another CP length by one to use first type subframes which are basic subframes. In this case, the frame having a CP length corresponding to ¼ of a useful symbol length causes interference due to misalignment with a frame having another CP length. Accordingly, for frames having different CP lengths, a method for uplink alignment and downlink alignment without interference will be required.